Messier 71

This globular cluster is one of the smallest of its kind.

Distance

13,000 light-years

Apparent Magnitude

6.1

constellation

Sagitta

object type

Globular Cluster

This spectacular NASA/ESA Hubble Space Telescope image shows a bright scattering of stars in the small constellation of Sagitta (the Arrow). This is the centre of the globular cluster Messier 71, a great ball of ancient stars on the edge of our galaxy around 13 000 light-years from Earth. M71 is around 27 light-years across. Globular clusters are like galactic suburbs, pockets of stars that exist on the edge of major galaxies. These clusters are tightly bound together by their gravitational attraction, hence their spherical shape and their name: globulus means “little sphere” in Latin. Around 150 such globular clusters are known to exist around our Milky Way, each one of them containing several hundred thousand stars. Messier 71 has been known for a long time, having been first spotted in the mid eighteenth century by Swiss astronomer Jean-Philippe de Cheseaux. Cheseaux discovered a number of nebulae in his career, and also spent much time studying religion: one posthumously published work attempted to derive the exact date of Christ’s crucifixion from astronomical events noted in the Bible. Despite being a familiar object, Messier 71’s precise nature was disputed until recently. Was it simply an open cluster, a loosely bound group of stars? This was for many years the dominant view. But in the 1970s, astronomers came to the view that it is in fact a relatively sparse globular cluster. The stars in Messier 71, as is usual in such clusters, are relatively old, at around 9 to 10 billion years, and consequently are low in elements other than hydrogen and helium. This picture was created from images taken with the Wide Field Channel of the Advanced Camera for Surveys on Hubble. It is a combination of images taken through yellow (F606W — coloured blue) and near-infrared (F814W — coloured red) filters. The exposure times were 304 s and 324 s respectively. The field of view is about 3.4 arcminutes across.

ESA/Hubble and NASA

Ever since it was discovered in 1746 by the Swiss astronomer Jean-Philippe Loys de Chéseaux, the nature of M71 has been the subject of some questioning. Over the centuries, it has been classified as both an abnormally dense open star cluster and as an unusually loose globular cluster.

Because M71 does not seem to have a high concentration of stars at its center, and because the elemental composition of the stars it contains suggests that they are younger than most globular clusters, M71 was initially believed to be an open cluster. In the 1970s, astronomers examined the relationship between the brightnesses and temperatures of the stars in the cluster. They found a characteristic trend among the stellar population that is unique to globular clusters, so M71 was reclassified as a young, loosely concentrated globular cluster — one of the smallest of its kind.

This Hubble image of M71 is a composite of observations at visible and infrared wavelengths of light. M71 is located roughly 13,000 light-years from Earth in the constellation Sagitta. It has an apparent magnitude of 6.1 and appears as a faint patch of light with a pair of binoculars. The cluster is best observed during September.

For more information about Hubble’s observations of M71, see:

Messier 71: An Unusual Globular Cluster

This star chart for M71 represents the view from mid-northern latitudes for the given month and time.

Image courtesy of Stellarium

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